Psoriasis is a chronic skin disease which afflicts about three percent of the U.S. population and has an incidence of one to three percent worldwide. It is characterized by dry, scaling plaques and often causes physical disfigurement and disability. There is no known cure for the disease and at present, no treatments which cause long-term remission. A variety of methods have been developed for reducing the severity of symptoms or inducing short-term remission. Several of these methods involve the use of light, at a variety of wavelengths, alone or in combination with topical or systemic pharmaceuticals.
The majority of sufferers generally report lessened symptoms during the summer months, presumably due to increased exposure to sunlight. Ultraviolet B light, one component of sunlight, has been used in conjunction with topical ointments such as extracts of coal tar as a treatment strategy. A more elaborate technique, referred to as PUVA, involves ultraviolet A light in conjunction with oral psoralen compounds. During PUVA treatment, patients are exposed to ultraviolet A light in a box similar to a tanning booth after ingesting the psoralen drug. Relief is temporary and treatment must be repeated after several months. This treatment has recently been associated with significantly higher rates of skin cancer.
Localized hyperthermia has also produced temporary clearing of psoriatic plaques. Several heat delivery systems have been investigated including infrared radiation from a sunlamp, ultrasound generation and hot water contact. Ultrasound hyperthermia provides the most uniform depthwise distribution of tissue heating.
Lasers have been employed in a variety of fundamentally different treatment approaches. These approaches include Ultraviolet delivery, photodynamic therapy and surgical excision.
Nitrogen lasers or excimer lasers have been used to replace the UV lamp in conventional light treatment approaches. This modification has not led to any reported benefits.
A tunable dye laser is used in photodynamic therapy to activate a photosensitive drug. Once activated the drug causes rapid cell death. The drug is administered systemically and serious side effects have been associated with its use.
Lasers have also been used as surgical tools in the treatment of psoriasis. A CO.sub.2 or argon laser is used to burn away the psoriatic plaque. Surgical techniques involve considerable expense and require protracted healing time. There is also a substantial risk of scarring or postsurgical infection.
Laser photocoagulation and selective photothermolysis are well known. In photocoagulation, individual vessels are visually targeted. U.S. Pat. No. 4,917,486, Raven et al., describes a photocoagulation apparatus for ophthalmological use. Infrared light, which can be effectively filtered without affecting the operator's view, is used for safety reasons. U.S. Pat. No. 4,829,262, Furumoto, describes a tunable dye laser particularly suited to selective photothermolysis. The output wavelength is tunable to match some spectral property of the target tissue and the laser pulse duration is tailored to the size of the target. Efforts to perform selective photothermolysis of blood vessels are typically carried out in the visible region of the spectrum (typically around 577 nm) because hemoglobin absorbs very well in that range. Pulsed lasers (typically flashlamp-pumped dye lasers) have been successfully employed to close down unwanted vessels under the skin surface in the treatment of portwine stain birthmarks. The poor transmission of visible light through skin tissue makes it difficult to close deep vessels using a pulsed dye laser and repeated treatments are required.
Psoriatic skin tissue is created by the body at ten times the rate of normal healthy skin. This results in an epidermis which may become much thicker than normal, e.g. 0.5-0.7 mm. The underlying dermal papillae become elongated and capillaries proliferate. Although there is no general agreement among physicians as to the importance of microvessels in the development, persistence or recurrence of psoriasis, it has been suggested that eliminating these excess microvessels may provide long-term skin clearing. The elongated dermal papillae which contain the capillaries may extend as much as one millimeter below the skin surface. Conventional pulsed dye lasers operating in the visible range do not have adequate tissue penetration to reach these deep microvessels.
Attempts to clear psoriatic plaques using pulsed dye lasers have met with limited success. The laser fluence required is at or above the level normally considered to cause scarring in healthy skin. In one recent study, Archives of Dermatology 128:853-855 (1992), some improvement was obtained in slightly more than half the treated patients.
It is therefore an object of the present invention to provide an improved method for treating psoriatic plaques using selective photothermolysis to destroy underlying blood vessels at depths of up to one millimeter without causing thermal damage to surrounding tissue. Summary of the Invention
The above object has been achieved in a method which selectively destroys subsurface microvessels with a pulsed infrared laser beam. A pulsed infrared laser light source, with an output wavelength in the range of 700 nm to 1100 nm is placed above a portion of the lesion and a laser pulse is triggered. The pulse duration is in the range of 200 microseconds to 20 milliseconds, with longer pulse durations targeting larger vessels. The laser pulse delivers a treatment fluence of 5 0to 50 joules per square centimeter at the skin surface. Skin transmission is high at these wavelengths, as is the relative absorption of blood in comparison with surrounding tissue. While the absolute absorption of blood is very low at these wavelengths, it still absorbs much more strongly than the surrounding tissue. Blood is therefore selectively heated causing damage to the endothelial cells lining the vessel wall. The target vessels cease to carry blood after illumination and are gradually removed in the same way a bruise is cleared. One or more laser pulses may be used to treat a specific portion of the plaque. The laser source is then repositioned over an untreated area and the process repeated until the entire lesion has been treated.
In the preferred embodiments, a semiconductor diode laser or semiconductor diode laser array with an output wavelength of approximately 800 nm is used. A pulse duration in the millisecond range is used to target vessels with diameters of 10 to 300 microns. Each laser pulse illuminates an area of approximately one-half square centimeter. No post-treatment dressing is required.
An advantage of the method of the present invention is that it selectively destroys blood vessels underlying psoriatic lesions at depths of up to one millimeter without excessive heating of the surrounding tissue.